1. Field of the Invention
This invention relates to an improved nursery or greenhouse container or pot. More specifically, the invention relates to an apparatus and method for vertical air-root-pruning of plants grown in containers.
2. Description of the Prior Art
Plants have long been grown in pots in greenhouses and homes. The practice of producing large numbers of plants out-of-doors in containers has developed primarily since the early 1950's. The container nursery industry began in southern California and spread across the southern states. The #10 food can with a few holes punched in the bottom was widely used and soon became known as the "one gallon container". During the sixties and seventies, the container nursery industry increased rapidly for several reasons: (1) landscape plants grew at a faster rate in containers than in the field; (2) turnover time decreased; (3) the root system of the plant remained undisturbed; thus, planting could be done anytime, not just during the early spring as with bare root or balled-in-burlap nursery stock; and (4) ease of display and handling made container grown plants attractive to the consumer.
However, development of the container nursery industry was not without problems. The complex nutritional requirements of plants grown in containers took years to define. In addition, growing media was refined until the quality of plant growth in containers attained that of field grown plants. The medium for the container evolved from field soil, to mixes of field soil and compost, to soilless mixes with far greater pore space for providing oxygen to the root system.
Numerous articles have been written and a common topic at gatherings of nurserymen is root development, especially of woody plants, in containers. As a root grows from a cutting or seedling in a container, its path is outward (towards the side of the container) and downward. When the root reaches the side of a round container it follows the contour and generally after one half to one full circle, reaches the bottom where it may continue to elongate and circle, sometimes for five or more revolutions, all of which is considered to be deleterious to the plant.
In "Growth of Carissa grandiflora `Boxwood Beauty` in varying media, containers, micronutrient levels", The Florida Nurseryman, 17(4):12-13, 43 (1972), Whitcomb tried placing holes in the sides of containers to improve root growth but without success (see also U.S. Pat. No. 3,785,088). Later studies with tree seedlings grown in square bottomless containers on a raised wire bench showed that air-pruning was effective in stopping root elongation and wrapping at the bottom of the container and, at the same time, in stimulating lateral branch root development following the death of the root tip, Davis et al, "Effects of Propagation Container Size on Development of High Quality Tree Seedling", Proc. Int. Plant Prop. Soc.; 25:448:453 (1975). More recent studies showed that Bur oak trees (Quercus macrocarpa) grew larger and developed a more fibrous root system in a square bottomless container than in a conventional round container of the same volume, Hathaway and Whitcomb, "The Effects of Root Malformation during Propagation on Growth and Survival of Bur Oak", Research Report P-760, Oklahoma Agricultural Experimental Station, Oklahoma State University, pages 33-34 (1977). Unfortunately, growing plants in bottomless containers on raised wire benches in neither practical nor economical. Birchell and Whitcomb, "Effects of Container Design on Root Development and Regeneration", Research Report P-760, Oklahoma Agricultural Experimental Station, Oklahoma State University, pages 39-45 (1977) compared the growth of birch trees grown in bottomless containers with vertical ribs on the sides. The vertical ribs stopped the circling or the wrapping of the roots of a fine, fibrous rooted species such as the birch. In addition, when the vertical ribs were present, there was no advantage to removing the bottom of the container for air-pruning. Dickinson and Whitcomb, "Effects of Container Design on Root Quality", Research Report P-760, Oklahoma Agricultural Experimental Station, Oklahoma State University, pages 35-36 (1977) tried placing ribs across the bottom of a round container and vertical ribs one fourth to one half the height of the sidewall of the container in order that the container could be nested for stacking and shipping. Japanese black pine (Pinus thunberi) and bald cypress (Taxodium distishum) trees were grown in these containers for one growing season. The vertical ribs in the lower one fourth or one half of the container were effective in stopping circling of the pine roots; however, the more coarsely rooted cypress either bent the rib and continued to circle or was stopped by the rib from circling but continued to elongate creating a "tangled ball of string" effect.
Dickinson and Whitcomb, "The Effects of Spring Versus Fall Planting on Establishment of Landscape Plants", S.N.A. Nursery Research Journal 4(1):9-19 (1977) observed that the roots of container grown plants that developed following planting were extensions of roots that were already present in the container at the time of planting, and were not "new" roots. They suggest that the number of root tips present at planting time may be very important to the rapid establishment and frequently the survival of the container grown plants in the landscape.
These studies showed that the root system of a plant grown in a container could be improved (a) as in the case of bottomless containers on a wire bench and (b) that vertical ribs on the inside of the container could improve the root structure of fine, fibrous rooted plants, but only worsened the problem for strong, coarsely rooted plants. Also, neither improvement was practical for the production of nursery stock on a commercial scale.